SCSI (small computer system interface) bus protocol is used in facilitating communication between one or more initiators and one or more targets. Generally speaking, a computer is used as an initiator and a peripheral device, such as a disk drive, is used as a target. The initiator typically initiates an operation while the target performs the operation. In conjunction with providing the communication path, single-ended interfaces are commonly incorporated in both initiator and target SCSI devices where the maximum cumulative cable length between the devices is 6.0 meters. Differential interfaces are commonly incorporated in both initiator and target SCSI devices where the maximum cumulative cable length is 25 meters, although differential interfaces can be used in applications requiring less than 6.0 meters. A single-ended interface is involved with the use of one signal in representing each control and data signal sent over the SCSI bus, while a differential interface is involved with the use of two signals (positive and negative polarity signals) in representing each control and data signal sent over the SCSI bus.
The SCSI bus for carrying differential signals cannot be directly coupled to a SCSI bus that carries single-ended signals. In order to provide desired communication between the different cables connected to initiators and targets, single-ended signals must be translated to differential signals and differential signals must be translated to single-ended signals. In one application, it is necessary to test the interfaces of single-ended initiator(s) and differential target(s). Interconnection between single-ended initiator(s) and differential target(s) can establish interface functionality without additional test equipment, typically SCSI simulators. That is, it would be desirable to test such interfaces by connecting the targets and initiators together using a translator intermediate the initiators and targets. In connection with providing the requisite translation, it is essential that, once priority is established by the system for a particular SCSI control signal, the translator maintain control for the initiator or target that was granted priority, until that initiator or target becomes inactive and releases priority.
U.S. Pat. No. 4,864,291 to Korpi issued Sept. 5, 1989 and entitled "SCSI Convertor" describes an apparatus for converting between single-ended and differential SCSI signals. The converter logic appears to be involved with the process for establishing priority when more than one device seeks access to the SCSI bus, during the arbitration phase, at the same time. This converter appears to become involved before a determination is made as to which one of the initiators and targets will be awarded bus priority. This configuration is relatively complicated and discloses different logic for avoiding latch-up conditions, i.e., circuitry that converts between single-ended and differential signals may become latched and then cannot be released, such as when more than one device seeks access to the SCSI bus at the same time.